In this work Spin-Polarized X-ray Photoelectron Diffraction (SPXPD) is studied on the basis of multiple scattering theory in the relativistic theoretical framework. This approach is inevitable to study XPD and SPXPD spectra from heavy elements (Z > 50), where relativistic effects play some important roles. In the K-edge excitation spin-orbit interaction for photoelectrons is quite important to produce spin-polarization of photoelectrons by use of circularly polarized light, and that this effect has influence on the branching ratio of multiplet splitting in photoemission spectra. In the case of linearly polarized light this phenomenon is expected to be small. We, however, expect that we can see finite contribution in the forbidden direction. In that direction one of important relativistic effects, spin flip scatterings excited by linearly polarized X-ray photons is remarkable not only in heavy atoms but in light atoms. [DOI: 10.1380/ejssnt.2009.115]